usb: gadget: hid: Fix static variable usage

If we have multiple instances of hid function, each of
them may have different report descriptor, also their
length may be different.

Currently we are using static hidg_desc varable which
is being filled in hidg_bind(). Then we send its content
to host in hidg_setup() function. This content may
have been already overwriten if another instance
has executed hidg_bind().

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>